The screening and inspection of cargo, including packages, baggage, mail, pallets, containers, materials and discrete objects such as machinery or vehicles, in customs and security applications is commonly conducted using X-ray imaging systems. Where palletized cargo is handled or materials are consolidated onto pallets or into large containers, it is often advantageous to inspect or screen the pallets and containers using X-ray imaging systems without the need to break-down these structures to constituent elements. Similarly it is often advantageous to screen large discrete objects, groups of objects or bulk materials without break-down or disassembly in customs, security or non-destructive testing applications. Typically, for cargo or other items and materials of pallet or similar dimensions, X-ray imaging systems using X-ray generators in the range 180 kV to 450 kV are used to acquire projection linescan X-ray images in either single view or, increasingly, dual view forms. Such imaging systems may operate with dual energy imaging capability and provide certain limited information regarding the effective atomic number and density or thickness of the materials imaged. However, current commercial technology is inherently limited in the discrimination of objects and materials, as the projection image information provided is derived from the interaction of a polychromatic X-ray beam with all the materials along the line of sight path through the object between the X-ray source and each individual detector. The resulting X-ray projection image therefore has limited utility as multiple objects or features in a given region of the image are not individually resolved with respect to depth, but rather overlay one another and form image clutter. The recent trend of using dual view X-ray linescan systems, which provide an additional near orthogonal view, partially alleviates this clutter limitation. However, for many cargos, objects or materials, it remains unlikely that the contents, components or composition of interest will be sufficiently isolated in one of these two views for ready visual identification or to effectively support detection or analysis using automated systems designed to flag threat, weapon, contraband items or other characteristics. Similarly, both visually and using image analysis methods designed to synergistically exploit the independent information from dual views, it remains very difficult to adequately identify, detect and analyze target objects and characteristics. For both visually inspected and automated detection or analysis systems, a high probability of detection or discrimination and a low false alarm rate cannot typically be achieved using single or dual view X-ray linescan systems at an adequate level of performance for most applications.
For some screening and inspection applications, such as aviation hold baggage security, the state of the art for operational systems has advanced to the use of computed tomography systems to provide high resolution three-dimensional, volumetric information and has largely overcome the limitations mentioned above. Typically, helical scan CT systems using a rotating gantry mounting an X-ray source and one or more detector arrays are employed. Commonly multiple detector rows form partial rings over a detector arc of typically approximately 60 degrees. The bag or item to be inspected passes on a conveyor linearly through the aperture in the rotating gantry and screening can be conducted with high detection and low false alarm performance at many hundreds of bags per hour in the most advanced systems. However, this technology is not readily scalable to the dimensions of palletized or containerized cargo. The mechanical designs required to provide an adequate gantry aperture rotating at sufficient angular velocity and to mount high voltage and high power X-ray generator systems are cost prohibitive.
Similarly, significant advances have been made in the development of stationary CT systems for hold baggage inspection and security in various forms using arrays of switchable X-ray sources placed around the inspection tunnel in a plane essentially normal to the direction of conveyor travel. Detector arrays may be interspersed in the same image plane or just out of plane and acquisition of data suitable for CT reconstruction is accomplished by fast switching between X-ray sources and readout of pre-determined detector elements. However, these system configurations are also not readily scalable to the characteristics of palletized cargo as a primary cost driver for large scale screening systems is the X-ray generator subassembly. Arrays of high voltage, high power, and switchable X-ray sources are not commercially available and, if developed, would likely be prohibitively expensive for these applications.
In current security and customs X-ray screening, imaging, the information visualized, detection and analysis capability is tied to the system type used. For example, for palletized cargo, the current state of widely deployed technology allows the operator to view projected transmission images in typically one or two viewing perspectives at most, with the imaging and analysis limitations associated with X-ray transmission techniques discussed above. For hold and carry-on baggage, more sophisticated imaging techniques are available, as described above, providing 3D volumetric CT imaging where imaging information may be viewed by various means including in surface rendered form, slice form, readily manipulated by rotation about an arbitrary axis and displayed and visualized using many other tools. The volumetric 3D data provided by such systems also supports effective automated image analysis methods which can highlight threats, target objects and other characteristics. However, the CT techniques provided for hold and carry-on baggage have not been widely adopted for large and palletized cargo primarily because the physical sensor systems needed to provide such information have not been available in a sufficiently cost-effective form with suitable operational characteristics and performance.
Embodiments of the current invention circumvent the design limitations of current commercial CT screening and inspection technology and provide cost and operationally effective three-dimensional volumetric imaging information for pallet and larger objects beyond the dimensions of currently available technology. Other embodiments provide improved visualization of two and three-dimensional datasets from CT and transmission X-ray imaging to facilitate identification of objects, and analysis of contents, components or compositions of interest.